2G/3G Network Architecture
This page provides a detailed look at the architecture of 2G (GSM) and 3G (UMTS) networks, including all major network elements and their functions.
Understanding Network Standards
3GPP (Third Generation Partnership Project) is a collaboration of telecommunications standards organizations that develops protocols for mobile telecommunications.
Release Versions:
- R4 (Release 4): Introduced MSC Server/MGW separation (2001)
- R99 (Release 99): First 3G UMTS standard (2000)
- R5 (Release 5): IMS introduction (2002)
- R6-R18: Progressive enhancements
Major Network Equipment Vendors:
- Nokia (Finland) - formerly Nokia Siemens Networks
- Ericsson (Sweden) - telecom infrastructure leader
- Huawei (China) - global network equipment provider
- ZTE (China) - telecommunications equipment manufacturer
- Samsung (South Korea) - 5G infrastructure
- Cisco (USA) - IP/packet core solutions
- Ciena (USA) - optical transport
Note: Operators typically deploy multi-vendor networks - e.g., Nokia RAN + Ericsson Core + Huawei Transport.
2G GSM Network Architecture
Complete Network Diagram
Network Elements Explained
1. Mobile Station (MS)
Components:
- ME (Mobile Equipment): The physical phone
- SIM (Subscriber Identity Module): Contains subscriber information
Functions:
- Transmit/receive radio signals
- Store subscriber identity (IMSI)
- Authentication
2. Base Station Subsystem (BSS)
BTS - Base Transceiver Station
Function:
- Radio interface with mobile stations
- Transmits and receives on Um interface
- Modulation/demodulation of signals
- Channel coding/decoding
- Encryption/decryption
Location: Cell towers
BSC - Base Station Controller
Function:
- Controls multiple BTS (typically 10-100)
- Radio resource management
- Handover decisions
- Power control
- Frequency hopping
Interfaces:
- Abis: BSC ↔ BTS
- A: BSC ↔ MSC
3. Network Switching Subsystem (NSS)
MSC - Mobile Switching Center
The "brain" of the circuit-switched network.
Functions:
- Call routing and switching
- Mobility management
- Handover control (between BSCs)
- Interconnection to other networks
- Charging and billing data collection
Interfaces:
- A: MSC ↔ BSC
- E: MSC ↔ MSC (for handovers)
GMSC - Gateway MSC
Functions:
- Gateway to external networks (PSTN/ISDN)
- Routes incoming calls to correct MSC
- Queries HLR for routing information
- International gateway
VLR - Visitor Location Register
Database containing:
- Temporary subscriber data
- Location area information
- Active subscribers in MSC area
- Copy of HLR data for roaming users
Functions:
- Manages roaming subscribers
- Faster access than querying HLR
- Usually co-located with MSC
HLR - Home Location Register
Central database containing:
- Permanent subscriber data
- IMSI (International Mobile Subscriber Identity)
- MSISDN (Mobile phone number)
- Subscribed services
- Current location (VLR address)
- Authentication keys
Functions:
- Subscriber profile management
- Location tracking
- Service authorization
AuC - Authentication Center
Functions:
- Subscriber authentication
- Generate authentication triplets (RAND, SRES, Kc)
- Stores authentication keys (Ki)
- Usually co-located with HLR
EIR - Equipment Identity Register
Database containing:
- IMEI (International Mobile Equipment Identity)
- Stolen/invalid device blacklist
- Approved device whitelist
4. GPRS Network Elements (2.5G Data)
SGSN - Serving GPRS Support Node
Functions:
- Packet routing and transfer
- Mobility management for data
- Session management
- User authentication
- Charging data collection
Analogous to: MSC (but for packet data)
GGSN - Gateway GPRS Support Node
Functions:
- Gateway to external packet networks (Internet)
- IP address allocation
- Firewall and filtering
- QoS enforcement
Analogous to: GMSC (but for packet data)
Network Planes
Like PSTN, a 2G mobile network separates its functions into two distinct planes that run in parallel over the same physical infrastructure.
┌──────────────────────────────────────────────────────────────────┐
│ 2G Network Planes │
│ │
│ SIGNALING PLANE (Control) VOICE PLANE (Bearer/User) │
│ ───────────────────────── ───────────────────────── │
│ Call setup & teardown Actual voice samples │
│ Location updates PCM-encoded audio streams │
│ Authentication 64 Kbps circuit per call │
│ Subscriber management Established only after │
│ SMS delivery signaling succeeds │
│ │
│ Protocol: SS7 (MAP, ISUP, …) Protocol: TDM / PCM │
└──────────────────────────────────────────────────────────────────┘
The Analogy: PSTN vs 2G
Just as the traditional PSTN uses:
- Signaling network (SS7) to set up/tear down calls and manage routing
- Voice network (TDM circuits) to carry the actual conversation
2G GSM does the same:
- Signaling plane — built on SS7, carries MAP/ISUP messages for call control, location updates, and authentication
- Voice plane — carries the encoded voice over dedicated 64 Kbps TDM circuits (E1/T1 links) once a call is established
Which Elements Live in Which Plane?
| Network Element | Signaling Plane | Voice Plane | Notes |
|---|---|---|---|
| HLR | ✅ | ❌ | Pure database — only sends/receives SS7 MAP messages |
| VLR | ✅ | ❌ | Subscriber registry — signaling only |
| AuC | ✅ | ❌ | Authentication triplet generation — no media |
| EIR | ✅ | ❌ | IMEI check — signaling only |
| SMS Center (SMSC) | ✅ | ❌ | SMS is a signaling-layer service (SMS over MAP) |
| STP | ✅ | ❌ | SS7 router — pure signaling traffic |
| MSC | ✅ | ✅ | Handles both: call control logic + circuit switching |
| GMSC | ✅ | ✅ | Both: routes calls (signaling) + connects circuits (voice) |
| BTS / BSC | ✅ | ✅ | Radio control messages + encoded voice frames |
Key insight: HLR, VLR, AuC, EIR, SMSC, and STP never touch voice. They only exchange signaling messages. Voice circuits are set up by the MSC and flow through BTS → BSC → MSC → GMSC → PSTN, bypassing these elements entirely.
Plane Separation in Action — Call Setup
3GPP Release 4 — Explicit Plane Separation
In 3G Release 4, 3GPP made this implicit separation explicit and physical by splitting the MSC into two dedicated nodes:
2G (Monolithic MSC) 3G R4 (Split)
┌──────────────────┐ ┌─────────────────┐ Signaling Plane
│ │ │ MSC Server │ (SS7 / BICC / SIP)
│ MSC │ →→→ └────────┬────────┘
│ (Signaling + │ │ Mc (H.248)
│ Voice mixed) │ ┌────────┴────────┐ Voice Plane
│ │ │ MGW │ (RTP / TDM)
└──────────────────┘ └─────────────────┘
This split allowed operators to scale signaling and media independently, and to deploy MGWs at the edge while keeping MSC Servers centralized.
2G Network Interfaces
| Interface | Between | Purpose |
|---|---|---|
| Um | MS ↔ BTS | Radio/Air interface |
| Abis | BTS ↔ BSC | Control and traffic |
| A | BSC ↔ MSC | Voice and signaling |
| E | MSC ↔ MSC | Handover signaling |
| C | MSC ↔ HLR | Location/subscriber info |
| D | VLR ↔ HLR | Location updates |
| Gb | BSC ↔ SGSN | GPRS data |
| Gn | SGSN ↔ GGSN | GPRS backbone |
3G UMTS Network Architecture
3GPP Release 4 (R4) Architecture
3GPP Release 4 introduced a major architectural change: separation of media and control planes.
Why R4 was revolutionary:
- Split MSC into MSC Server (signaling) and MGW (media)
- Enabled IP-based transport for voice
- Paved the way for IMS and VoLTE
- Released in 2001, widely deployed 2005-2010
Key Differences from 2G
1. Radio Access Network (UTRAN)
NodeB
- Equivalent to BTS in 2G
- Handles radio transmission/reception
- Supports WCDMA technology
- Higher data rates
RNC - Radio Network Controller
- Equivalent to BSC in 2G
- Controls multiple NodeBs
- Radio resource management
- Soft handover coordination
- More intelligence than BSC
Soft Handover: Mobile can connect to multiple NodeBs simultaneously
2. 3GPP Release 4 Innovations
Separation of Media and Control
Traditional 3G (R99):
Release 4 (R4):
Benefits:
- Scalability: Scale media and signaling independently
- Flexibility: Deploy MGW closer to edge
- Cost reduction: Use IP transport
- Future-proof: Easier migration to IMS
MSC Server
Functions:
- Call control and signaling
- Mobility management
- No media handling
- Uses BICC (Bearer Independent Call Control)
MGW - Media Gateway
Functions:
- Media conversion (circuit ↔ packet)
- Codec transcoding
- Echo cancellation
- Tone generation
- Controlled by MSC Server
BICC - Bearer Independent Call Control
- Separates call control from bearer
- Based on SS7/ISUP
- Allows for more flexible bearer allocation
- Enables efficient use of IP transport
3. Packet-Switched Domain
SGSN (3G Enhanced)
Additional functions over 2G:
- Higher data rates (HSPA)
- QoS management
- Supports both 2G and 3G access
GGSN (3G Enhanced)
- Same basic function as 2G
- Higher throughput support
- Better QoS support
3G Network Interfaces
| Interface | Between | Purpose |
|---|---|---|
| Uu | UE ↔ NodeB | WCDMA air interface |
| Iub | NodeB ↔ RNC | Control and traffic |
| Iu-CS | RNC ↔ MSC Server | Circuit-switched signaling |
| Iu-PS | RNC ↔ SGSN | Packet-switched data |
| Mc | MSC Server ↔ MGW | Media gateway control |
| Gn | SGSN ↔ GGSN | GPRS backbone |
Supporting Network Elements
STP - Signaling Transfer Point
Functions:
- SS7 signaling router
- Routes signaling messages between network elements
- Provides network redundancy
- Load balancing
Protocol: SS7 (Signaling System 7)
CDS - Call Detail Server / CDR
Functions:
- Collects Call Detail Records (CDR)
- Billing and charging information
- Traffic analysis
- Network planning data
Data Collected:
- Call duration
- Caller and called numbers
- Time and date
- Cell/location information
- Service type
Network Technology Comparison
GSM vs CDMA (2G)
| Feature | GSM (TDMA) | CDMA |
|---|---|---|
| Multiplexing | Time slots | Unique codes |
| Capacity | Moderate | Higher |
| Coverage | Wider | Better in-building |
| Handoff | Hard handoff | Soft handoff |
| SIM Card | Yes | No (built-in ESN) |
| Global Roaming | Better | Limited |
| Power Control | Less critical | Critical |
Call Flow in 2G/3G (Circuit Switched)
Mobile Originated Call
Key Points
- Dedicated circuit established for entire call duration
- Guaranteed bandwidth for voice quality
- MSC coordinates the entire call setup
- HLR validates subscriber permissions
- GMSC bridges to external networks
GPRS/EDGE Data Flow (Packet Switched)
Technologies Deep Dive
GSM Technology Stack
Physical Layer (L1):
- Modulation: GMSK (Gaussian Minimum Shift Keying)
- Channel coding
- Burst formatting
Data Link Layer (L2):
- LAPDm protocol
- Error detection and correction
- Flow control
Network Layer (L3):
- CM (Connection Management): Call control
- MM (Mobility Management): Location updates, authentication
- RR (Radio Resource): Channel allocation
TDMA Frame Structure
- 8 time slots per TDMA frame
- Each user gets one time slot
- Up to 8 simultaneous users per frequency
- Each slot: 0.577 ms
CDMA Spread Spectrum
In CDMA, all users transmit on the same frequency simultaneously, but each has a unique code.
Key Concepts:
- Spreading: Narrow-band signal spread over wide frequency
- PN Codes: Pseudo-random noise codes for each user
- Correlation: Receiver uses same code to extract signal
- Near-Far Problem: Requires strict power control
Advantages:
- Soft capacity limit
- Soft handoff (connected to multiple cells)
- Better security
- Efficient spectrum use
Summary
2G GSM Architecture
- BSS: BTS + BSC (radio access)
- NSS: MSC, VLR, HLR (core network)
- GPRS: SGSN, GGSN (packet data)
- Circuit-switched voice
- TDMA multiplexing
3G UMTS (Release 4) Architecture
- UTRAN: NodeB + RNC (radio access)
- Core Network: MSC Server + MGW (separated)
- BICC: Bearer-independent signaling
- Packet-switched data (SGSN/GGSN)
- WCDMA technology
- Higher data rates
Network Equipment Vendors by Element
| Network Element | Common Vendors | Notes |
|---|---|---|
| BTS/NodeB/eNodeB | Ericsson, Nokia, Huawei | Radio access equipment |
| BSC/RNC | Ericsson, Nokia, Huawei | Radio controllers |
| MSC/MSC Server | Ericsson, Nokia, Huawei | Core switching |
| MGW | Nokia, Ericsson, Ribbon | Media gateways |
| HLR/HSS | Oracle, Nokia, Ericsson | Subscriber databases |
| SGSN/GGSN | Cisco, Ericsson, Nokia | Packet core |
| IMS Core | Ericsson, Nokia, Mavenir | VoLTE/IMS |
| Transmission | Ciena, Nokia, Huawei | Fiber/microwave |
Multi-Vendor Networks:
Example: Airtel Network (India)
├─ RAN: Nokia + Ericsson + Huawei
├─ Core: Ericsson (Circuit) + Cisco (Packet)
├─ IMS: Ericsson
├─ Transport: Ciena + Nokia
└─ HSS: Oracle
Key Network Elements
| Element | Function | Network |
|---|---|---|
| BTS/NodeB | Radio transmission | 2G/3G |
| BSC/RNC | Radio controller | 2G/3G |
| MSC | Call switching | 2G/3G |
| MGW | Media gateway | 3G (R4+) |
| HLR/HSS | Subscriber database | All |
| SGSN | Packet routing | 2G/3G data |
| GGSN | Gateway to Internet | 2G/3G data |
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